The present invention relates generally to the fields of amplifier and speaker apparatus and more particularly to a method and system for improving the average electrical efficiency of a switching power amplifier in a speaker apparatus or system.
Distributed paging systems utilize a group of individual speakers that are distributed throughout an area or facility that are connected together electrically for the purpose of delivering paging announcements, background music or other information throughout the area or facility. The two types of distributed paging systems primarily in use today are centrally amplified and self-amplified systems.
Centrally amplified paging systems use amplifiers with maximum output levels typically limited to the range of 25 to 100 Vrms. Centrally amplified paging systems typically use passive speakers with locally mounted transformers that step down the maximum output levels from the central amplifier to levels more compatible with the passive speakers. Centrally amplified paging systems allow many passive speakers to be connected in parallel without the need to make complicated calculations or connect the passive speakers in ways that would overload the central amplifiers. However, system expansion in small power increments is difficult to achieve since central amplifiers have fixed maximum power outputs. The need to exceed the maximum load capability of a centralized amplifier will require equipment replacement with a higher-powered amplifier model. This causes additional system expense and the next higher power level may be significantly more than necessary which adds unnecessary cost to the paging system.
Self-amplified paging systems do not use central amplifiers. In a self-amplified paging system, each speaker has a local amplifier built into its enclosure. These amplifiers have traditionally been based on analog designs. A plurality of self-amplified speakers are connected to a remote power supply that is capable of powering the plurality of speakers. Typically, it is relatively simple to connect a large number of speakers in parallel in a self-amplified speaker system to a remote power supply. The total power of the paging system can be finely scaled with no unnecessary system power left in reserve as sometimes happens with central amplifier systems.
Self-amplified speaker system typically uses four conductors: (1) two conductors to carry power from the remote power supply to the amplifier; and (2) two conductors to carry the audio signal to the input of the amplifier. Typically, the remote power supply for a self-amplified speaker system is 24 VDC and the audio signal voltage level is comparable to that of a conventional analog telephone system. However, the voltage drop in the two conductors that connect to the remote power supply for a self-amplified speaker system can be a problem with background art self-amplified systems. The maximum distance between the power supply and the self-amplified system is limited by the combination of the gauge of wire used (i.e., due to its associated resistance) and the typical current requirements of the local amplifier.
In addition, background art implementations of self-amplified power amplifiers typically use linear Class A or Class AB type power amplifier topologies. Both class A and Class AB power amplifiers utilize a pair of output transistors to produce a power amplified output signal from the input signal. In Class A amplifiers, both output transistors are continuously conducting or “on” throughout the entire input signal cycle. The Class A configuration produces a very linear output, provides the least distortion of the input signal but typically can only provide an average electrical efficiency of 20%. The average electrical efficiency of a device can be characterized in terms of the ratio of output power produced-to-input power required by the device or system.
In Class AB amplifiers, both output transistors conduct for more than half but less than the entire input signal cycle. Efficiencies of about 50% are possible with the Class AB power amplifier configuration. However, as further discussed below, disadvantages to the use of these types of analog power amplifier configurations include: (1) relatively poor average electrical efficiency; (2) large input power supply requirements; (3) excessive heat generation and (4) physically large hardware implementations.
When used to amplify musical and voice programming, the average electrical efficiency of Class A and Class AB power amplifier configurations typical of background art self-amplified power amplifiers is only around 30%. As a result, for musical and voice programming applications, 70% of the input power is being wasted and results in excessive heat generation by the device or system.
In low output power applications (e.g., below 5 Watts), the relatively low average electrical efficiency of the Class A and Class AB amplifiers discussed above is not problematic. However, at higher levels of output power the background art self-amplified speaker apparatus or systems require a large amount of input power supply current to produce the required output power. Thus, the low average electrical efficiency of Class A and Class AB power amplifiers is a disadvantage when, as in musical and voice programming applications, large amounts of input power are required to produce a desired level of output power.
In addition, to support the large input power supply requirements, Class A and Class AB power amplifiers may also require a larger gauge power supply wiring. Typically, Class A and Class AB amplifier circuits require a minimum of 15 VDC to operate properly. For a given gauge of power supply wiring, the length of the wire between the power supply and the amplifier/speaker apparatus must be restricted in order to limit resistive power loss to an acceptable level above 15 VDC. That is, due to resistive power loss, the terminal voltage can drop below the 15 VDC level and, as a result, the apparatus or system will shut down and produce no output power. Even if the speaker device or system does not completely shut down due to these resistive losses, the acoustical output from the speaker may be highly distorted and unintelligible.
In addition, many paging systems cover large areas and the distances between adjacent speaker/amplifier apparatus and from the apparatus to the remote power supply can require large distances of interconnecting cable. If the lengths of the cable connecting the apparatus and its associated power supply are very limited in length in comparison to the total linear distance of cable required to connect all apparatus, then more power supplies must be installed. Additionally the acceptable location of these additional power supplies becomes somewhat specific due to the length limitation and the required locations may not be suitable for power supply installation because of its lack of AC power receptacles, lack of suitable mounting surfaces or exposure to damage or tampering. Making an area suitable for mounting a power supply can be costly, inconvenient and time consuming. Therefore, a system that allows significantly longer cable lengths while performing as well or better than the current art mitigates many of these problems is economically valuable to the owner of the system.
Another disadvantage to the use of prior art Class A and Class AB amplifiers is a limitation on the system amplifier power output rating. Because of the high current consumption of linear power amplifiers, it is impractical to create remotely powered loudspeaker systems that have in excess of 30 watts maximum power output to the internal speaker. In the past manufacturers have tried to remedy this problem by running the system on 48 VDC to halve the required current in an effort to reduce the required interconnecting wire gauge between the remote power supply and the speaker system and keep the wire lengths to usable run lengths. Obviously this creates other problems as some of the system may be run on 24 VDC and cannot share the same power supply and power supply wiring for existing units, which complicates system expansion and coverage. Also the higher-powered systems over 30 watts output could not practically dissipate the heat required from a physical size standpoint and fit inside the speaker system package. This limitation is removed from the switching amplifier approach by its 2 times average efficiency advantage.
Further, increases in the ambient temperature in the device or system due to the heat generated by Class A and Class AB amplifiers can require the use of larger heat sinks to keep operating temperatures at a safe level that ensures proper circuit operation. Of course, larger heat sinks have the disadvantage of increasing the weight, cost and physical size of the self-amplified power amplifier apparatus or system. In consideration of the disadvantages of background art Class A and Class AB amplifiers discussed above, there is a need in the art of self-amplified power amplifiers to improve average electrical efficiency and reduce input power supply requirements, heat generation and physical size.
Furthermore, many paging systems are installed out of doors and as such must be weatherproof. In weatherproof reentrant horn loudspeakers the enclosure encasing the class D amplifier will be completely sealed to prevent water infiltration and as such no cooling ventilation of the amplifier enclosure is possible. This lack of ventilation accelerates the ambient temperature rises within the enclosure when a class A or AB amplifier is used. Additionally, the material that forms the enclosure is often a non-thermally conducting material like plastic providing no means of radiating the built up heat from the enclosed space. The heat build up can cause overheating and in response the amplifier may protect itself by ceasing operation temporarily, interrupting the operation of the paging system or the amplifier may complete fail. These conditions can present themselves even at low to moderate continuous output power levels, like those used for low volume background music in a paging system. This is especially true in ambient environments subject to high temperatures like those found outdoors in the all parts of the country during the summer, or where the speaker/amplifier apparatus is subject to direct sunlight.